It is often desired to print conductive inks on polymer film substrates. Such polymer film substrates may then be applied to glass panels, for example. Such glass panels may be used to form solar panels. However, it can be difficult to print conductive inks on film substrates. In particular, the ink often takes a long time to dry or cure which slows down the production process. If the ink is not sufficiently dried, the ink may smear or not properly adhere to the substrate.
The most common methods of printing inks on a substrate are screen printing methods onto glass or integrated circuit boards. Such methods utilize thicker layers of ink and consequently greater amounts of ink. Such screening methods produce good results but at very slow production rates as the required curing times can be 3 to 20 minutes.
A particular type of screen printing is with a rotogravure. This rotogravure screen printing is used to apply conductive ink on thin polymer films for use in thin film solar panels. The expense for this type of printing is very high and processing runs are approximately 50 feet per minute. Moreover, rotogravure screen printing requires two steps, which adds to the overall cost.
There are several problems with screen printing conductive ink on polymer film substrates including printability problems, poor adhesion, and slow drying.
It is very difficult to obtain desired printing results using engraved cells on the rotogravure. The engraved cells do not lay the ink in a continuous line. Instead, a pattern of disconnected diamond shapes or islands is printed as shown in FIG. 1. Such a disconnected pattern does not provide a continuous electrical circuit. The desired effect of the printing process is a well defined straight printed line to provide good electrical circuit conductivity.
Another problem of prior art conductive inks is poor adhesion to the film substrate. The ink may print well onto the film, but the ink then separates from the film. Slowing down the process to allow greater curing or drying time helps alleviate this problem somewhat, but the process becomes too slow for practical production processes. Furthermore, such inks do not react well to later electroplating processes used when preparing solar panels, for example.
Another problem is that poor ink circulation in the holding tank can cause scumming, where the ink separates easily. Thus the ink needs to be constantly stirred or circulated to keep a homogenous consistency.
It is therefore desired to formulate a conductive ink that will provide a well defined printed line or other desired shape and will bind or adhere to a film substrate such that the conductive ink can tolerate an electroplating process